TII!«;  OPERATION  OF LAND   POWER   PLANTS      279
arising from the unbalanced mass of the ring overcomes the spring and allows
the ring to strike a trigger which releases the main-valve operating gear.
Probably the best type of main governor is that where the main valve
is controlled by oil (fig. 13). This system by oil control lends itself very
well to the requirements of the emergency governors set to shut down the
main plant when the speed exceeds about 12 per cent above the normal.
Condensers.-—The testing of condensers, especially where sea water
is used as circulating water, is important. Testing for and replacing faulty
tubes may cout as much per annum as the whole of the electrical
plant.
The testing of eondcn-	_4—~i
sers is important because	^      '        \
of the havoc that may be
wrought  in the plant  if
salt water is permitted to
get; into the boiler water.
It  has  been found that,    j'
should    a    leak   develop   •[
which will cause a salinity    L
of  more  than  about   12
grains of salt: per gallon, the
water should be turned to
waste until such, time as
the condenser can be tested
and the leak, stopped.
Fiff  14.—Kmcrtfcncy Governor
There are two simple
methods of testing the eon-
dcnsate for salt when the plant is on load. One of these consists of drawing
a sample of water and analysing it in the usual manner for salinity by the
chemical process (Vol. IV). The other method is an electrical one, An
apparatus can be arranged to give an alarm directly the salinity reaches a
predetermined value. The apparatus is simple, and has been at work for
some time, and up to the present it has given very good results (fig. 12).
When it comes to finding the leak in the condenser, the work usually
has to be carried out during the periods of light load, when the turbine plant
can be shut down. The air-pump connections are either blanked off, or
a valve is provided that can be shut. The manhole is then opened at the
top of the condenser, which allows the condenser body to be filled with
fresh water. The manhole doors in the condenser end plates are also opened
and an inspection is made of all the nipples in the condenser tube plates
until the leak is discovered. It is quite a common thing to find that a
number of tubes begin to fail simultaneously, and though each allows only
a minute leak, together they are dangerous.
On a big condensing plant this test may take as long as six hours. It
is by far the quickest method, however, that has yet been tried, and is the
most certain in its results.